Rolling bearing unit with permanent magnet, coil and back yoke for sensing rotational speed of automotive wheel

A rolling bearing unit for sensing rotational speed of a rotatable member, comprising:

• said rotatable member which is of a magnetic material and has a shaft terminating in an end face,
• a first rotating ring means secured to the rotatable member and having a first raceway on a peripheral surface thereof,
• a second stationary ring means having a second raceway on a peripheral surface thereof so as to oppose the first raceway with a space therebetween,
• a plurality of rolling bodies provided in the space between the first raceway and the second raceway so that the first and second ring means may rotate freely relative to each other,
• an annular pulse rotor fixedly attached to the shaft of said rotatable member proximate said end face and adapted to rotate in a coaxial relationship with the rotatable member, and
• a sensor for sensing said rotational speed of said rotatable member and being supported on the second stationary ring means so as to axially face the pulse rotor, and comprising a pole piece of magnetic material with a first axial end face thereof facing the pulse rotor, a coil wound around the pole piece, a permanent magnet with a first axial end face in contact with a second axial end face of the pole piece, and a back yoke made of a magnetic material having a base end portion thereof in contact with a second axial end face of the permanent magnet and a tip end portion thereof arranged in proximity to and facing the end face of the shaft.